The invention relates to hydrophilic copolymers, built up from polymeric chains, which carry acidic and basic side groups, and to the use of these copolymers for hydrophilizing lithographic printing plates.
To produce offset printing plates, suitable layer supports are provided on one or both sides with a light-sensitive layer (resist layer), by means of which a printing image is generated by photomechanical means. After the printing image has been produced, the layer support carries the printing image areas and, in the image-free areas (non-image areas) forms at the same time the hydrophilic image background for the lithographic printing process.
A layer support for light-sensitive layers, suitable for producing lithographic plates, must therefore meet the following requirements. First, the parts of the light-sensitive layer applied to the layer support, which are relatively readily soluble after exposure, must be detachable easily and without residue from the support in a developing process to produce the non-image areas. The support bared in the non-image areas must be highly hydrophilic, i.e., have a high affinity to water, so that it can rapidly absorb and permanently hold water during the lithographic printing step and thus sufficiently repel the greasy printing ink. The light-sensitive layer must also have adequate adhesion to the layer support; even the printing areas of the layer, which have remained after exposure and development, must still adhere sufficiently strongly to the support in order to achieve a long print run.
Foils of aluminum, steel, copper, brass or zinc and also plastic films or paper can be used as the starting material for such layer supports. These raw materials are converted to layer supports for offset printing plates by a suitable treatment of their surface, such as graining, matte chromium-plating, superficial oxidation and/or application of an interlayer. Aluminum, which is probably the most widely used base material for offset printing plates nowadays, is superficially roughened by known methods such as dry brushing, wet brushing, sandblasting, chemical and/or electrochemical treatment or a combination thereof. To increase the abrasion resistance, the aluminum thus roughened can also be subjected to an anodic oxidation in order to build up a thin oxide layer.
In practice, the support materials, especially aluminum based, anodically oxidized support materials, are frequently subjected to a further treatment step before a light-sensitive layer is applied, for improving the layer adhesion, for increasing the hydrophilic character and/or for enhancing the developability of the light-sensitive layers. These include, for example, the methods known from DE-C-907,147 (=U.S. Pat. No. 2,714,066), DE-B 1,471,707 (=U.S. Pat. No. 3,181,461 and U.S. Pat. No. 3,280,734) or DE-A-2,532,769 (=U.S. Pat. No. 3,902,976), which disclose processes for hydrophilizing printing plate support materials based on aluminum which may have been anodically oxidized. These materials are treated with aqueous sodium silicate solution, without or with the use of electric current.
DE-A-1,134,093 (=U.S. Pat. No. 3,276,868) and DE-C-1,621,478 (=U.S. Pat. No. 4,153,461) have disclosed the use of polyvinylphosphonic acid or copolymers based on vinylphosphonic acid, acrylic acid and vinyl acetate for hydrophilizing printing plate support materials based on aluminum which may have been anodically oxidized. The use of salts of these compounds is also mentioned, but not specified in more detail.
The use of complex fluorides of titanium, zirconium or hafnium according to DE-B-1,300,415 (=U.S. Pat. No. 3,440,050) also leads to additional hydrophilizing of aluminum oxide layers on printing plate support materials.
In addition to these most widely known hydrophilizing methods, the use of the following polymers in this field of application has also been described. For example, in DE-B-1,056,931, the use of water-soluble, linear copolymers based on alkyl vinyl ethers and maleic anhydrides in light-sensitive layers for printing plates is described. In addition, those copolymers in which the maleic anhydride component has been reacted partially or fully with ammonia, an alkali metal hydroxide or an alcohol are also mentioned.
DE-B-1,091,433 has disclosed hydrophilizing of printing plate support materials based on metals by means of film forming organic polymers, such as polymethacrylic acid or sodium carboxymethyl-cellulose or sodium hydroxyethylcellulose for aluminum supports or by means of a copolymer of methyl vinyl ether and maleic anhydride for magnesium supports.
According to DE-B-1,173,917 (=UK 907,719), water-soluble polyfunctional synthetic amino/urea/aldehyde resins or sulfonated synthetic urea/aldehyde resins, which are cured on the metal support for conversion into a water-insoluble state, are used for hydrophilizing printing plate support materials of metals.
To produce a hydrophilic layer on printing plate support materials, a) an aqueous dispersion of a modified urea/formaldehyde resin, of an alkylated methylolmelamine resin or of a melamine/formaldehyde/polyalkylenepolyamine resin and b) an aqueous dispersion of a polyhydroxy compound or polycarboxy compound, such as sodium carboxymethylcellulose are applied, according to DE-B-1,200,847 (=U.S. Pat. No. 3,232,783), successively to the support and the substrate c) thus coated is then treated with an aqueous solution of a salt of Zr, Hf, Ti or Th.
In DE-B-1,257,170 (=U.S. Pat. No. 2,991,204) a copolymer which, in addition to units of acrylic acid, acrylate, acrylamide or methacrylamide, also contains Si-trisubstituted vinylsilane units, is described as a hydrophilizing agent for printing plate support materials.
DE-A-1,471,706 (=U.S. Pat. No. 3,298,852) has disclosed the use of polyacrylic acid as a hydrophilizing agent for printing plate support materials of aluminum, copper or zinc.
According to DE-C-2,107,901 (=U.S. Pat. No. 3,733,200), the hydrophilic layer on a printing plate support material is formed from a water-insoluble hydrophilic homopolymer or copolymer of acrylate or methacrylate, having a water absorption of at least 20% by weight.
In DE-B-2,305,231 (=UK 1,414,575) hydrophilizing of printing plate support materials is described, in which a solution or dispersion of a mixture of an aldehyde and a synthetic polyacrylamide is applied to the support.
DE-A-2,308,196 (=U.S. Pat. No. 3,861,917) has disclosed hydrophilizing of roughened and anodically oxidized aluminum printing plate supports with ethylene/ maleic anhydride copolymers or methyl vinyl ether/maleic anhydride copolymers, with polyacrylic acid, carboxymethylcellulose, sodium poly(vinylbenzene-2,4-disulfonic acid) or polyacrylamide.
In DE-B-2,364,177 (=U.S. Pat. No. 3,860,426), a hydrophilic adhesion layer for aluminum offset printing plates is described, which layer is located between the anodically oxidized surface of the printing plate support and the light-sensitive layer and which, in addition to a cellulose ether, also contains a water-soluble salt of Zn, Ca, Mg, Ba, Sr, Co or Mn. The layer weight of cellulose ether in the hydrophilic adhesion layer is 0.2 to 1.1 mg/dm.sup.2, and the same layer weight is also quoted for the water-soluble salts. The mixture of cellulose ether and salt is applied to the support in aqueous solution, if appropriate with the addition of an organic solvent and/or of a surfactant.
According to U.S. Pat. No. 3,672,966, anodically oxidized aluminum surfaces are, before or during the sealing with hot water, treated with aqueous solutions of acrylic acid, polyacrylic acid, polymethacrylic acid, polymaleic acid or copolymers of maleic acid with ethylene or vinyl alcohol in order to avoid seal deposits.
According to U.S. Pat. No. 4,049,746, hydrophilizing agents for printing plate support materials contain salt-like reaction products of water-soluble polyacrylic resins having carboxyl groups and polyalkyleneimine/urea/aldehyde resins.
In UK 1,246,696, hydrophilic colloids such as hydroxyethylcellulose, polyacrylamide, polyethylene oxide, polyvinylpyrrolidone, starch or gum arabic, are described as hydrophilizing agents for anodically oxidized aluminum printing plate supports.
EP-B-0,149,490 describes, for hydrophilizing, compounds which additionally contain carboxyl groups or carboxylate groups, sulfo groups or hydroxyl groups, apart from amino groups. These compounds have a molecular weight of at most 1000.
The state of the art has also disclosed the use of metal complexes, which contain low-molecular ligands, for hydrophilizing printing plate support materials. Examples of such complexes are: complex ions of divalent or polyvalent metal cations and ligands, such as ammonia, water, ethylenediamine, nitric oxide, urea or ethylenediaminetetraacetate (DE-A-2,807,396 U.S. Pat. No. 4,208,212); iron cyanide complexes such as K.sub.4 (Fe(CN).sub.6) or Na.sub.3 (Fe(CN).sub.6) in the presence of heteropolyacids, such as phosphomolybdic acid or salts thereof, and of phosphates (U.S. Pat. No. 3,769,043 and/or U.S. Pat. No. 4,420,549); and iron cyanide complexes in the presence of phosphates and complex formers such as ethylenediaminetetraacetic acid for electrophotographic printing plates having a zinc oxide surface (U.S. Pat. No. 3,672,885).
In EP-A-0,069,320 (=U.S. Pat. No. 4,427,765), a process is described in which salts of polyvinylphosphonic acids, polyvinylsulfonic acids, polyvinylmethylphosphonic acids and other polyvinyl compounds are used as after treatment agents.
In DE-A-2,615,075 (=UK 1,495,895), a process for treating image-bearing offset printing plates with polyacrylamide or a mixture of polyacrylamide and polyacrylic acid is used.
In SU-A-647,142, a copolymer of acrylamide and vinyl monomers is used for hydrophilizing offset printing plates.
DE-C-1,091,433 describes a process for the after treatment of offset printing plate supports with polymers of methacrylic acid, methyl vinyl ether and maleic anhydride.
Acrylamide for the treatment of printing plate supports is also mentioned in DE-A-2,540,561.
For the same purpose, especially for improving the storage stability of printing plates, DE-A-2,947,708 describes, inter alia, Ni salt solutions of acrylamide and acrylic acid and also acrylamide and vinylpyrrolidone.
All the methods described above, are, however, affected by greater or lesser disadvantages, so that the support materials thus produced frequently no longer meet the current demands of offset printing with respect to developer resistance, hydrophilic properties, free running behavior and steady print runs. Thus, after the treatment with alkali metal silicates, which lead to good developability and hydrophilic character, a certain deterioration in the storage stability of light-sensitive layers applied thereto must be accepted, and the print run of a printing plate thus after treated drops drastically.
Although the complexes of the transition metals in principle promote the hydrophilic character of anodically oxidized aluminum surfaces, they have the disadvantage of being very readily soluble in water, so that they can easily be removed during the development of the layer with aqueous developer systems which recently increasingly contain surfactants and/or chelate formers which have a high affinity to these metals. As a result, the concentration of the transition metal complexes on the surface is reduced to a greater or lesser extent, and this can lead to a weakening of the hydrophilic effect.
In the treatment of supports with water-soluble polymers without scope for anchorage, their high solubility, especially in aqueous-alkaline developers such as are predominantly used for developing positive-working light-sensitive layers, also leads to a marked weakening of the hydrophilizing effect.
Monomeric hydrophilic compounds such as those described, for example, in EP-B-0149,490, quite generally have the disadvantage of being washed away relatively rapidly from the bared non-image area surface during the development and printing process and losing their hydrophilic effect, since the anchorage points in the surface are insufficient.
The combination of a mixture of a water-soluble polymer, such as a cellulose ether, and a water soluble metal salt leads, since the layer weights and hence the layer thickness are selected at a relatively high level (see DE-B-2,364,177), to reduced adhesion of the resist layer, and this can manifest itself, for example, in parts of the developer fluid undermining image areas during the development.